Method for forming electrical contact to the optical coating of an infrared detector using conductive epoxy

ABSTRACT

This is a system and method of forming an electrical contact to the optical coating of an infrared detector using conductive epoxy. The method may comprise: forming thermal isolation trenches 22 and bias contact vias 23 in a substrate 20; depositing a trench filler 24 in the thermal isolation trenches 22; depositing conductive epoxy 50 into the bias contact vias 23; replanarizing; depositing a common electrode layer 31 over the thermal isolation trenches 22 and vias 23; depositing an optical coating 26 above the common electrode layer 31; mechanically polishing a backside of the substrate 20 to expose the trench filler 24 and conductive epoxy 50; depositing a contact metal 34 on the backside of the substrate 20; etching the contact metal 34 and the trench filler 24 to form pixel mesas of the contact metal 34 and the substrate 20.

This is a divisional of application Ser. No. 08/397,706, filed Mar. 1,1995, now U.S. Pat. No. 5,552,326.

CROSS-REFERENCE TO RELATED APPLICATION

The following coassigned patent applications and issued patent arehereby incorporated herein by reference:

    ______________________________________                                        Serial No.    Filing Date                                                                             TI Case No.                                           ______________________________________                                        08/223,073    04/04/94  TI-18726                                              08/223,088    04/04/94  TI-18727                                              ______________________________________                                        Patent No.    Filing Date                                                                             Inventor                                              ______________________________________                                        5,466,332     04/04/94  Owen et al.                                           ______________________________________                                    

FIELD OF THE INVENTION

This invention generally relates to infrared (IR) detector arrays andmeans of fabrication.

BACKGROUND OF INVENTION

The novel IR devices and fabrication processes to be described arerelated to the types of IR detector arrays recorded in (1) U.S. Pat. No.4,080,532, Hopper, Mar. 1978; (2) U.S. Pat. No. 4,745,278, Hanson, May1988; (3) U.S. Pat. 4,792,681, Hanson, Dec. 1988; (4) "LOW-COST UNCOOLEDFOCAL PLANE ARRAY TECHNOLOGY", by Hanson, Beratan, Owen and Sweetser;presented Aug. 17, 1993 at the IRIS Detector Specialty Review; (5)cross-referenced patent application Ser. No. 08/223,087, filed Apr. 04,1994, (6) cross-referenced patent application Ser. No. 08/223,088, filedApr. 04, 1994, and (7) cross-referenced patent application Ser. No.08/223,073, filed Apr. 04, 1994.

The physical requirements of uncooled arrays and a description offabrication processes are covered in some detail in the abovereferences. A line scanner may contain from several hundred to a fewthousand and an area imager several thousand to tens of thousandindividual picture elements (pixels.) Each of these pixels consists of acapacitor that has a heat (IR intensity) sensitive dielectric such asbarium strontium titanate (BST.) The electronic connections to thesecapacitors are greatly simplified if one of the capacitor terminals ismade common to all. Previously described in the references are methodsto attach hundreds to tens of thousands of electrical connectionsbetween the other isolated terminals of the pixel capacitors and theelectronic sensing circuitry housed on an external integrated circuit(IC.) In addition, the pixel capacitors must be thermally isolated fromeach other while having one terminal connected to all the other commonterminals.

The common connection to one side of the pixel capacitors consists of afront side thin film referred to as the optical coating. This may be acomposite of a plurality of thin films having the desired physicalproperties, such as IR transparency, electrical conductivity, thermalconductivity, etc. The thicker heat sensitive dielectric substrate inthis case can be barium-strontium-titanate (BST) which is a ceramic likematerial.

SUMMARY OF THE INVENTION

Most of the previous inventions are concerned with how to achievethermally isolated pixels and attach them to a companion siliconintegrated circuit without damage to the fragile IR sensing arraycircuitry. The IR sensing side of the array contains the previouslymentioned optical coating. This typically consists of three or fourlayers. The outermost layer is a semitransparent thin metal film such asnichrome (NiCr). The underlying wavelength tuning layer has a thicknessof an odd number of quarter wavelengths at the desired IR wavelength.This tuning layer is transparent to IR and may be parylene. Theunderlying metal completes the optical filter properties and may also bethe sole electrical contact for a common electrical connection to allthe pixels. Typically this layer is a thicker film of NiCr. An improvedstructure, such as described in the references, may have a more ruggedfourth layer. Sometimes this fourth layer results in a structurereferred to as an elevated optical coating.

Although much detail and many variants of the pixel isolation andconnection processes are described in the references, not mentioned isthe method whereby the common electrode of the optical layer iselectrically connected to a voltage or current biasing supply.Heretofore this has been accomplished in a very primitive and unreliablemanner by physically scraping away the top two coatings of the opticallayer at the periphery of the IR sensing army and attaching a free wirewith electrically conducting epoxy.

This invention describes novel means of effecting this common electrodebiasing connection in a more production worthy and reliable manner. Thisis a system and method of forming an electrical contact to the opticalcoating of an infrared detector using conductive epoxy. The method maycomprise: forming thermal isolation trenches in a substrate; depositinga trench filler in the thermal isolation trenches; depositing aconductive epoxy in vias at the periphery of the substrate;replanarizing; depositing a common electrode layer over the thermalisolation trenches; depositing an optical coating above the commonelectrode layer; mechanically polishing a backside of the substrate toexpose the trench filler; depositing a contact metal on the backside ofthe substrate, wherein said contact metal forms a bias contact with saidconductive epoxy and said common electrode; and etching the contactmetal and the trench filler to form pixel mesas of the contact metal andthe substrate. This system and method may be applied to uncooled as wellas cooled infrared detectors.

DESCRIPTION OF THE DRAWINGS

This invention can be best understood by reference to the followingdrawing(s), in which:

FIG. 1 shows a cross sectional sketch of an array after bias contactvias and thermal isolation trenches have been patterned and etched;

FIG. 2 shows a cross sectional sketch of an array after the bias contactvias have been filled with a conductive epoxy and the thermal isolationtrenches have been filled with a trench filler and replanarized;

FIG. 3 shows the optical coating applied over the sensing and commonbiasing contact area;

FIG. 4 depicts the mechanically thinned composite array with biasingcontact area and a layer of contact metal on the backside of the array;

FIG. 5 shows a cross sectional sketch of the sensing array after thetrench filler has been removed;

FIG. 6 show the final array structure after bonding to the companion IC.

Corresponding numerals and symbols in different figures refer tocorresponding parts unless otherwise indicated.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

One preferred embodiment will be described with the aid of FIGS. 1-6 andTABLE 1. The figures have exaggerated layer thicknesses for descriptivepurpose and are not to actual or relative scale to each other. In thisembodiment, the BST substrate 20 is patterned from the front oroptically sensitive side of the sensing array by the use of a laser tovaporize the substrate thereby forming thermal isolation trenches 22between the thermally sensitive picture elements (pixels). As shown inFIG. 1, the vias 23 for the biasing contacts are formed in the samemanner. These vias 23 are removed from the IR sensing area and havemultiple locations around the border of the array. Although shown inthese figures to be the same size as the thermal isolation trenches 22,these vias 23 may be larger if desired.

FIG. 2 exhibits the structure after several processing steps have beencompleted from FIG. 1. Trenches 22 have been filled with parylene 24 andthe surface has been mechanically polished to planarize the front sideof the IR detector. In addition, the bias contact vias have been filledwith a conductive epoxy 50. The conductive epoxy may then be planarized.Of course, the order of deposition of the trench filler and conductiveepoxy may be switched. Additionally, both materials may be planarized atone time after deposition.

After planarization, a three level optical coating 26 is deposited overall the pixels and bias contact vias 23 as shown in FIG. 3. Thematerials and thicknesses are shown in TABLE 1. The common electrode inthis case is 31 and provides the functions of one side of the pixelcapacitors and the electrical contact in the bias contact areas.

In FIG. 4, the substrate 20 has been thinned from the back side bymechanical polishing means to expose the parylene 24 and conductiveepoxy 50.

FIG. 5 shows the array after removal of the parylene from the thermalisolation trenches. In addition, FIG. 5 shows the deposition of the backside contact metal 34 which typically consists of the alloys shown inTABLE 1. This metal 34 acts as a backside contact metal in the thermalsensing areas, as well as a bias contact metal in the bias contactareas.

After the use of standard photolithography, etching and cleaningtechniques, and bumpbonding to the IC, the structure shown in FIG. 6results. The capacitor pixel mesas 20 are defined by the optical coating26, the thermally sensitive insulator 20 and the contact metal 34. ICbonding may now be performed to all the pixel mesas and the biasingpads. This establishes an electrical connection for the common pixelelectrode bias voltage through 34 to 50 to 31. The IC elements are alsodescribed in TABLE 1.

                                      TABLE 1                                     __________________________________________________________________________    ID#                                                                              Description  Material (dim)                                                                            Alternates                                        __________________________________________________________________________    20 Thermally sensitive                                                                        Barium strontium                                                 substrate    titanate                                                      22 Isolation trenches                                                                         Laser vaporized                                                                           Ion milled                                        23 Bias contact vias                                                                          Laser vaporized                                                                           Ion milled                                        24 Trench filler                                                                              Parylene    Photoresist, PMMA, epoxy                          26 Optical coating                                                                            3 layered   1/4 IR wavelength                                 28 Transparent coat                                                                           NiCr 50 A   25-100 A                                          30 1/4 wavelength                                                                             Parylene 1.4 μm                                                                        1/4 wavelength                                       separator coat           at desired IR                                     31 Electrical conducting                                                                      NiCr 1000 A 500-2000 A                                           coat                                                                       34 Backside electrical                                                                        Multiple alloys suitable                                                                  4-layer composite of 15-                             contact      for IC bonding.                                                                           60 μm In                                                                   0.5-1.5 μm Au                                                              0.5-1.5 μm NiCr                                                            0.2-1.0 μm TiW                                 50 Bias contact material                                                                      Conductive Epoxy                                              76 Ohmic connection for                                                                       TiW                                                              pixels                                                                     77 IC contact mesa for                                                                        Photosensitive                                                                            PMMA                                                 sensing connection                                                                         polyimide, Dupont                                                             2734                                                          78 IC via for sensing                                                            circuit                                                                    80 IC processor Si or GaAs                                                    82 IC contact mesa for                                                                        Photosensitive                                                                            PMMA                                                 bias connection                                                                            polyimide, Dupont                                                             2734                                                          84 IC Ohmic connection for                                                                    TiW                                                              bias connection                                                            86 IC via for bias circuit                                                    92 IC bias contact pad                                                                        Conductive metal                                              __________________________________________________________________________

Whether the isolation trenches are formed by laser vaporization, ionmilling or other means, similar trenches may be patterned at theperiphery of the array at the same time. All variants and combinationsof these structures are considered within the scope of this invention.

While this invention has been described with reference to illustrativeembodiments, this description is not intended to be construed in alimiting sense. It should be stressed again that the biasing vias shownin the illustrations were drawn for convenience and ease of descriptionas if they were isolation trench type geometries. These vias are wellremoved from the sensing area and may be much larger or have many moreto cover a more extensive area than shown in the figures. In someembodiments of these novel IR sensors, thermal isolation of the pixelcapacitors is performed by partial etching from one side followed byetching to completion on the other side as has been described in thereferences. All of these methods can have the biasing vias patterned asa normal part of the process flow. In addition, this invention is not tobe limited to uncooled infrared detectors. For example, this inventioncould be easily incorporated into the normal process flow for cooledinfrared detectors as well.

Various modifications have already been described but othermodifications and combinations of the illustrative embodiments, as wellas other embodiments of the invention, will be apparent to personsskilled in the art upon reference to the description. It is thereforeintended that the appended claims encompass any such modifications orembodiments.

We claim:
 1. An infrared sensing device, said device comprising:anoptical coating; a common electrode connected to said optical coating;thermal isolation mesas in a sensing area in said device and below saidcommon electrode; bias contact areas around a periphery of said sensingarea; a conductive epoxy connected to said common electrode.
 2. Thedevice of claim 1, wherein said conductive epoxy connects said device toa bias contact pad on an integrated circuit.
 3. The device of claim 1,wherein said conductive epoxy connects said common electrode to anintegrated circuit by a contact metal layer.
 4. The device of claim 1,wherein said optical coating is a three layered optical coating.
 5. Thedevice of claim 4, wherein said three layer optical coating includes:anelectrical conducting coating; a 1/4 wavelength separator coating on topof said electrical conducting coating; and a transparent coat on top ofsaid separator coating.
 6. The device of claim 1, wherein said thermalisolation mesas is within a thermally sensitive substrate of bariumstrontium titanate.
 7. The device of claim 1, wherein said commonelectrode is Au.
 8. The device of claim 1, wherein said common electrodeis NiCr.
 9. The device of claim 1, wherein said common electrode is TiW.10. An infrared detector connected to an integrated circuit (IC), saiddetector comprising:an optical coating; a common electrode connected tosaid optical coating; thermal isolation mesas in a sensing area in saiddetector and below said common electrode; bias contact areas around aperiphery of said sensing area; a conductive epoxy connecting saidcommon electrode to said IC; and a contact metal layer connecting saidthermal isolation mesas to said IC.
 11. The detector of claim 10,wherein said conductive epoxy connects said detector to a bias contactpad on said IC.
 12. The detector of claim 10, wherein said conductiveepoxy connects said common electrode to said IC by a contact metallayer.
 13. The detector of claim 10, wherein said optical coating is athree layered optical coating.
 14. The detector of claim 13, whereinsaid three layer optical coating includes:an electrical conductingcoating; a 1/4 wavelength separator coat on top of said electricalconducting coating; and a transparent coat on top of separator coating.15. The detector of claim 10, wherein said thermal isolation mesas iswithin a thermally sensitive substrate of barium strontium titanate. 16.The detector of claim 10, wherein said common electrode is Au.
 17. Thedetector of claim 10, wherein said common electrode is NiCr.
 18. Thedetector of claim 10, wherein said common electrode is TiW.